RFR: 8366154: Validate thread type requirements in debug commands

[Kerem Kat]

On Mon, 1 Sep 2025 21:54:13 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Prevents segmentation faults during `gdb` sessions. The crashes were caused by the `ResourceMark` constructor being called on a native thread, which is not supported. This happened when invoking debug commands that require a `Thread` or `JavaThread` context from an incorrect thread type.
>> 
>> ### Solution
>> 
>> This change introduces `onThread()` and `onJavaThread()` helper methods to the `Command` class. These methods validate the thread context and ensure `ResourceMark` is only created when on a valid VM thread. All thread-dependent debug commands now use these guards to validate the context, printing a clear error and exiting gracefully upon failure.
>> 
>> ### Testing
>> 
>> Manually verified using `gdb` by calling the modified commands (`ps`, `universe`, `pns`, etc.) from different thread contexts (native, Java, and non-java threads) to ensure they fail gracefully with an error message instead of crashing the debug session.
>
> src/hotspot/share/utilities/debug.cpp line 341:
> 
>> 339:     }
>> 340:     return true;
>> 341:   }
> 
> I don't think we need this. The commands that require a JavaThread should be checking that directly themselves. Typically we assume/expect the person debugging to know what they are dealing with and use the appropriate commands.

I will remove this function and do the check in the 3 call sites of it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27033#discussion_r2315539345